1. Field of the Invention
This invention relates to photoresists, and more particularly to light sensitive photoresists that are based on diazoquinone sensitized polyamic acid.
2. The Prior Art
Photoresist compositions are well known in the art and comprise coatings deposited from a solution which when exposed to light of proper wave length are chemically altered in their solubility to certain solvents (developers). Two types are known, namely negative and positive photoresists. The negative acting resist is initially a mixture which is soluble in its developer but following exposure to activating radiation becomes insoluble in the developer thereby defining a latent image. Positive acting resists work in the opposite fashion, activating radiation making the resist soluble in the developer.
Positive working resists are known to the art and generally comprise a light sensitive compound in a film-forming polymeric binder. The resin binder most frequently used are the alkali soluble phenolformaldehyde resins. These materials are the product of a reaction of a phenol and formaldehyde under conditions whereby a thermoplastic (novolak) polymer is formed. Positive resists are prepared using the novolak type phenol formaldehyde resin by incorporating therein a photoactive compound, a so-called sensitizer, for instance, one of the group of the 4- or 5-substituted diazonaphthoquinone (1,2)-diazide-sulfonic acid esters.
In the prior art, the above described positive resists using novolak resins as a binder are most often used as masks to protect substrates from chemical etchants in the manufacture of semiconductors. In such manufacture, the photoresist is coated onto the surface of a semiconductor substrate and then imaged and developed to remove soluble portions thereof. The resist medium remaining on the surface of the substrate is then employed as a protective mask to facilitate the selective etching of the exposed portions of the substrate thereby defining a circuit pattern.
Etching of the substrate may be conventionally carried out by chemical treatment or by dry etching, e.g. reactive ion etching with chemically active gaseous e.g. fluorocarbon species formed by glow discharge. Chemical etching has the disadvantage that the purity and composition of the etchant must be carefully regulated and the duration of the etching must be carefully controlled to prevent under or over etching of the thin film. Reactive ion etching is a practical alternative to chemical etching. When using reactive ion etching the control problems associated with chemical etching are avoided and the etching equipment assures adequate process control for the most precise thin film patterns.
One of the problems associated with reactive ion etching, is that many resist materials cannot withstand the process intact and are eroded along with the substrate or flow due to reaction with the gaseous ions and the temperature of the semiconductor substrate (typically about 200.degree. C.) resulting in loss of pattern resolution. For example, novolak type phenol formaldehyde resins begin to flow at temperatures in excess of 120.degree. C. and will erode when struck by the gas stream generated during reactive ion etching.
Photosensitized polyamic acids have been disclosed by the art for the formation of relief patterns on a support, e.g. U.S. Pat. No. 4,451,551, wherein a coating film containing polyamic acid photosensitized with a compound having an amino group and an aromatic azide group is spin coated on a support, image-wise exposed to a source of UV radiation after which the unexposed portions are dissolved and removed with developing solution to form a negative relief pattern. After development, the polymeric relief pattern is a precursor of a heat resistant polyimide, and is converted to a heat resistant imide polymer by heat treatment at 150.degree.-300.degree. C. The resultant relief pattern is resistant to distortion when heated to 400.degree. C. for an hour and is disclosed as being useful as a dry-etching resistant photoresist.
Attempts have been made to prepare positive working polyamic acid based photoresist compositions using as the photosensitizer diazoquinone (1,2)-diazide-sulfonic acid esters. Such attempts have not been successful as the photoresist systems exhibit high dissolution rates for development in the alkaline solutions normally used as developers. The very high dissolution rates prevent adequate control over processes to obtain fine line configurations. Attempts to decrease the dissolution rate by increasing the concentration of the sensitizer in the photoresist, e.g. up to about 50% by weight, increases the optical density of the photoresist to the point where full penetration of the film thickness by a radiation source such as ultraviolet light is substantially unattainable.
There is, therefore, a need in the art for a system which will provide the control of the dissolution rate of diazoquinone sensitized polyamic acid based photoresists by alkaline developer solutions without reducing the sensitivity of the polyamic acid to ultraviolet light exposure so that excellent pattern delineation is obtained.